Power Mac PPC G5 start problems

I am a new to mac user, having switched from PCs about a month ago. I have a Power Mac G5 running OS X 10.5.8 that has start up problems - If I restart it will using do fine, but if I shut down, and power off, the thing has very annoying habits - I am so impressed with the G5 when it works that I bought another last week - it has no bad habits, but the first when powered on will display the white light, then either do nothing, or start the fans up, occasionally speeding them up to louder than the AC in the office. Nothing else happens - it does not boot up, just makes a noise with the fans.

Sometimes, if I leave it be for ten minutes or more, it will start up, do the chord sound, and within a couple of minutes I am up and running. However, on occasion, leaving for two hours gets nothing but varying fan speeds - no chord sound, no boot up.

The other G5 with the same OS X 10.5.8 works as I would expect. Power on, press the button, within a minute it is booted and running. I would like the other to do this, as it has a much larger hard drive and more memory, It appears to be getting worse - this morning I had to try five times, powering off and on, to get any response. Anyone any idea what is wrong?

I do not yet regard myself as experienced with Macs and do not know much about them, so simple answers would be appreciated.

Sounds very much like you have a dead power supply in the G5. Subject to them being the same model G5 you could swap the drives and memory about. There are five distinct PowerMac G5 models and some of the later ones use different memory, either PC-3200 or PC-4200.

Hang on to those original install discs like grim death! Using OS X.7 or later make a bootable USB thumb drive before running Installer!

My G5 seems to do the same, all attempts at finding the root cause have failed, my solution is not to turn it off, it sleeps and wakes on demand with no problems, so far after 6 months its running great and used every day :-) now with 3TB second drive.

Thanks for the replies - not particularly what I wanted to hear, . I will keep it running and see what happens. I will probably end up taking the drive out, and the memory and putting them in the other one.